diff --git a/README.md b/README.md
index 254b21e7..883253df 100644
--- a/README.md
+++ b/README.md
@@ -22,12 +22,14 @@ cargo install --path .
 ## Command-line usage
 
 ```bash
-mdtablefix [--wrap] [--renumber] [--in-place] [FILE...]
+mdtablefix [--wrap] [--renumber] [--breaks] [--in-place] [FILE...]
 ```
 
 - With file paths provided, the corrected tables are printed to stdout.
 - Use `--wrap` to also reflow paragraphs and list items to 80 columns.
 - Use `--renumber` to rewrite ordered lists with sequential numbering.
+- Use `--breaks` to normalize thematic breaks to a line of 70 underscores
+  (configurable via the `THEMATIC_BREAK_LEN` constant).
 - Use `--in-place` to overwrite files.
 - If no files are supplied, input is read from stdin and results are written to stdout.
 
diff --git a/src/lib.rs b/src/lib.rs
index f695f9b4..a9ec98b4 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -227,6 +227,14 @@ static BULLET_RE: std::sync::LazyLock<Regex> =
 static NUMBERED_RE: std::sync::LazyLock<Regex> =
     std::sync::LazyLock::new(|| Regex::new(r"^(\s*)([1-9][0-9]*)\.(\s+)(.*)").unwrap());
 
+/// Width of a normalised thematic break.
+/// The width used when rewriting thematic breaks.
+pub const THEMATIC_BREAK_LEN: usize = 70;
+
+static THEMATIC_BREAK_RE: std::sync::LazyLock<Regex> = std::sync::LazyLock::new(|| {
+    Regex::new(r"^[ ]{0,3}((?:[ \t]*\*){3,}|(?:[ \t]*-){3,}|(?:[ \t]*_){3,})[ \t]*$").unwrap()
+});
+
 /// Returns `true` if the line is a fenced code block delimiter (e.g., three backticks or "~~~").
 ///
 /// # Examples
@@ -541,6 +549,33 @@ pub fn renumber_lists(lines: &[String]) -> Vec<String> {
     out
 }
 
+#[must_use]
+/// Reformat thematic breaks as 70 underscores.
+///
+/// Thematic breaks are lines composed of three or more matching `-`, `_`, or
+/// `*` characters (optionally separated by spaces or tabs) with up to three
+/// leading spaces. Lines inside fenced code blocks are ignored.
+pub fn format_breaks(lines: &[String]) -> Vec<String> {
+    let mut out = Vec::with_capacity(lines.len());
+    let mut in_code = false;
+
+    for line in lines {
+        if FENCE_RE.is_match(line) {
+            in_code = !in_code;
+            out.push(line.clone());
+            continue;
+        }
+
+        if !in_code && THEMATIC_BREAK_RE.is_match(line.trim_end()) {
+            out.push("_".repeat(THEMATIC_BREAK_LEN));
+        } else {
+            out.push(line.clone());
+        }
+    }
+
+    out
+}
+
 #[must_use]
 pub fn process_stream(lines: &[String]) -> Vec<String> { process_stream_inner(lines, true) }
 
diff --git a/src/main.rs b/src/main.rs
index c3422459..43730dee 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -5,7 +5,7 @@ use std::{
 };
 
 use clap::Parser;
-use mdtablefix::{process_stream, process_stream_no_wrap, renumber_lists};
+use mdtablefix::{format_breaks, process_stream, process_stream_no_wrap, renumber_lists};
 
 #[derive(Parser)]
 #[command(about = "Reflow broken markdown tables")]
@@ -13,32 +13,44 @@ struct Cli {
     /// Rewrite files in place
     #[arg(long = "in-place", requires = "files")]
     in_place: bool,
+    #[command(flatten)]
+    opts: FormatOpts,
+    /// Markdown files to fix
+    files: Vec<PathBuf>,
+}
+
+#[derive(clap::Args, Clone, Copy)]
+struct FormatOpts {
     /// Wrap paragraphs and list items to 80 columns
     #[arg(long = "wrap")]
     wrap: bool,
     /// Renumber ordered list items
     #[arg(long = "renumber")]
     renumber: bool,
-    /// Markdown files to fix
-    files: Vec<PathBuf>,
+    /// Reformat thematic breaks as underscores
+    #[arg(long = "breaks")]
+    breaks: bool,
 }
 
-fn process_lines(lines: &[String], wrap: bool, renumber: bool) -> Vec<String> {
-    let mut out = if wrap {
+fn process_lines(lines: &[String], opts: FormatOpts) -> Vec<String> {
+    let mut out = if opts.wrap {
         process_stream(lines)
     } else {
         process_stream_no_wrap(lines)
     };
-    if renumber {
+    if opts.renumber {
         out = renumber_lists(&out);
     }
+    if opts.breaks {
+        out = format_breaks(&out);
+    }
     out
 }
 
-fn rewrite_path(path: &Path, wrap: bool, renumber: bool) -> std::io::Result<()> {
+fn rewrite_path(path: &Path, opts: FormatOpts) -> std::io::Result<()> {
     let content = fs::read_to_string(path)?;
     let lines: Vec<String> = content.lines().map(str::to_string).collect();
-    let fixed = process_lines(&lines, wrap, renumber);
+    let fixed = process_lines(&lines, opts);
     fs::write(path, fixed.join("\n") + "\n")
 }
 
@@ -72,18 +84,18 @@ fn main() -> anyhow::Result<()> {
         let mut input = String::new();
         io::stdin().read_to_string(&mut input)?;
         let lines: Vec<String> = input.lines().map(str::to_string).collect();
-        let fixed = process_lines(&lines, cli.wrap, cli.renumber);
+        let fixed = process_lines(&lines, cli.opts);
         println!("{}", fixed.join("\n"));
         return Ok(());
     }
 
     for path in cli.files {
         if cli.in_place {
-            rewrite_path(&path, cli.wrap, cli.renumber)?;
+            rewrite_path(&path, cli.opts)?;
         } else {
             let content = fs::read_to_string(&path)?;
             let lines: Vec<String> = content.lines().map(str::to_string).collect();
-            let fixed = process_lines(&lines, cli.wrap, cli.renumber);
+            let fixed = process_lines(&lines, cli.opts);
             println!("{}", fixed.join("\n"));
         }
     }
diff --git a/tests/integration.rs b/tests/integration.rs
index c883d41b..bdbea78e 100644
--- a/tests/integration.rs
+++ b/tests/integration.rs
@@ -1,7 +1,14 @@
 use std::{fs::File, io::Write};
 
 use assert_cmd::Command;
-use mdtablefix::{convert_html_tables, process_stream, reflow_table, renumber_lists};
+use mdtablefix::{
+    THEMATIC_BREAK_LEN,
+    convert_html_tables,
+    format_breaks,
+    process_stream,
+    reflow_table,
+    renumber_lists,
+};
 use rstest::{fixture, rstest};
 use tempfile::tempdir;
 
@@ -806,62 +813,68 @@ fn test_renumber_mult_paragraph_items() {
 }
 
 #[test]
-fn test_wrap_hyphenated_word() {
-    let line = format!("{} extremely-very-long-word end", "A".repeat(60));
-    let output = process_stream(&[line]);
-    assert_eq!(
-        output,
-        vec![
-            "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA".to_string(),
-            "extremely-very-long-word end".to_string(),
-        ]
-    );
+fn test_format_breaks_basic() {
+    let input = vec!["foo", "***", "bar"]
+        .into_iter()
+        .map(str::to_string)
+        .collect::<Vec<_>>();
+    let expected = vec![
+        "foo".to_string(),
+        "_".repeat(THEMATIC_BREAK_LEN),
+        "bar".to_string(),
+    ];
+    assert_eq!(format_breaks(&input), expected);
 }
+
 #[test]
-fn test_wrap_multiple_hyphenated_words() {
-    let line = format!("{} foo-bar baz-qux quux-corge end", "A".repeat(60));
-    let output = process_stream(&[line]);
-    assert_eq!(
-        output,
-        vec![
-            "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA foo-bar baz-qux"
-                .to_string(),
-            "quux-corge end".to_string(),
-        ]
-    );
+fn test_format_breaks_ignores_code() {
+    let input = vec!["```", "---", "```"]
+        .into_iter()
+        .map(str::to_string)
+        .collect::<Vec<_>>();
+    assert_eq!(format_breaks(&input), input);
 }
 
 #[test]
-fn test_wrap_hyphenated_word_at_boundary() {
-    let line = format!("{} extremely-very-long-word end", "A".repeat(55));
-    let output = process_stream(&[line]);
-    assert_eq!(
-        output,
-        vec![
-            "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA extremely-very-long-word"
-                .to_string(),
-            "end".to_string(),
-        ]
-    );
+fn test_format_breaks_mixed_chars() {
+    let input = vec!["-*-*-"]
+        .into_iter()
+        .map(str::to_string)
+        .collect::<Vec<_>>();
+    assert_eq!(format_breaks(&input), input);
+}
+
+#[test]
+fn test_format_breaks_with_spaces_and_indent() {
+    let input = vec!["  -  -  -  "]
+        .into_iter()
+        .map(str::to_string)
+        .collect::<Vec<_>>();
+    let expected = vec!["_".repeat(THEMATIC_BREAK_LEN)];
+    assert_eq!(format_breaks(&input), expected);
 }
 
 #[test]
-fn test_wrap_word_longer_than_width() {
-    let long_word = "a".repeat(90);
-    let output = process_stream(&[long_word.clone()]);
-    assert_eq!(output, vec!["a".repeat(80), "a".repeat(10)]);
+fn test_format_breaks_with_tabs_and_underscores() {
+    let input = vec!["\t_\t_\t_\t"]
+        .into_iter()
+        .map(str::to_string)
+        .collect::<Vec<_>>();
+    let expected = vec!["_".repeat(THEMATIC_BREAK_LEN)];
+    assert_eq!(format_breaks(&input), expected);
 }
 
 #[test]
-fn test_wrap_line_without_hyphenated_words() {
-    let line = format!("{} lorem ipsum dolor sit amet", "A".repeat(60));
-    let output = process_stream(&[line]);
+fn test_cli_breaks_option() {
+    let output = Command::cargo_bin("mdtablefix")
+        .unwrap()
+        .arg("--breaks")
+        .write_stdin("---\n")
+        .output()
+        .unwrap();
+    assert!(output.status.success());
     assert_eq!(
-        output,
-        vec![
-            "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA lorem ipsum dolor"
-                .to_string(),
-            "sit amet".to_string(),
-        ]
+        String::from_utf8_lossy(&output.stdout),
+        format!("{}\n", "_".repeat(THEMATIC_BREAK_LEN))
     );
 }